The present invention relates generally to methods and apparatus for semiconductor wafer processing, and specifically to methods of processing based on rapid thermal chemical vapor deposition (RTCVD).
Hemispherical grained silicon (HSG) has been found to be particularly advantageous in the production of ultralarge-scale integrated (ULSI) semiconductor devices, such as high-density DRAM. There is consequently a growing demand for selective deposition of HSG. Methods for promoting HSG formation are described by Watanabe et al., in xe2x80x9cAn Advanced Technique for Fabricating Hemispherical-Grained (HSG) Silicon Storage Electrodes,xe2x80x9d in IEEE Transactions on Electron Devices 42 (1995), pages 295-300; and by Sanganeria et al., in xe2x80x9cLow Temperature Silicon Epitaxy in an Ultrahigh Vacuum Rapid Thermal Chemical Vapor Deposition Reactor Using Disilane,xe2x80x9d in Applied Physics Letters 63 (1993), pages 1225-1227. These articles are incorporated herein by reference.
HSG deposition, like many of the steps in semiconductor wafer processing, is best performed under high vacuum. The presence of even very small concentrations of residual gases in an evacuated process chamber generally has deleterious effects on the selectivity of HSG formation. These constraints pose a particular challenge in high-performance integrated RTCVD processing, in which a number of process modules are typically connected together in a cluster tool configuration. High vacuum must generally be maintained not only in the RTCVD modules themselves, but also in other cluster tool components, such as load locks, transfer chambers and cleaning modules. In order to maximize up-time of the cluster tool, it is also desirable that the system be able to recover the required vacuum pressure rapidly after atmospheric exposure for cleaning or maintenance.
Many different methods and systems have been developed to reduce residual impurities, such as water vapor, in semiconductor process chambers. One such method is described, for example, by Moslehi, in xe2x80x9cLow-Temperature In-Situ Dry Cleaning Process for Epitaxial Layer Multiprocessing,xe2x80x9d in SPIE 1393 Rapid Thermal and Related Processing Techniques (1990), pages 90-108, which is incorporated herein by reference. Similarly, in U.S. Pat. No. 5,062,271, which is incorporated herein by reference, Okumura, et al., describe a method and apparatus for evacuation of a processing chamber using a turbomolecular pump in series with a heat exchanger cooled by a helium refrigerator. The heat exchanger freeze-traps certain gas molecules, particularly water vapor, which normally cannot be efficiently pumped by the turbomolecular pump.
It is an object of some aspects of the present invention to provide an improved method and apparatus for selective formation of HSG on a semiconductor wafer.
It is a further object of some aspects of the present invention to provide an improved vacuum system and method of pumping for RTCVD processes.
In preferred embodiments of the present invention, vacuum apparatus for a RTCVD process chamber comprises a turbomolecular pump and a water vapor pump, which communicate with the chamber via a vacuum isolation valve. The water vapor pump selectively removes water vapor from the chamber without pumping substantial amounts of process gases, which are pumped by the turbomolecular pump. The apparatus enables the chamber to rapidly reach a high vacuum with a reduced level of residual water vapor relative to the level achievable using a turbomolecular pump alone. Preferably, the base pressure that is achieved is less than 10xe2x88x927 torr, more preferably less than 10xe2x88x928 torr, and most preferably at or below 10xe2x88x929 torr.
In some preferred embodiments of the present invention, the water vapor pump comprises a cryogenic pump, which is preferably connected in series between the isolation valve and the turbomolecular pump. When the cryogenic pump becomes saturated, the isolation valve is preferably closed, and the turbomolecular pump is used to pump off water vapor that is released as the cryogenic pump is allowed to warm up.
In other preferred embodiments of the present invention, the water vapor pump comprises a getter-type pump, which is preferably connected in a sidestream configuration between the isolation valve and the turbomolecular pump. Preferably, the water vapor pump comprises a disposable getter, which is closed off and replaced when it becomes saturated. Alternatively, the getter may be regenerated by pumping it out through the backing pump with the isolation valve closed, as in the case of the cryogenic pump described above.
In some preferred embodiments of the present invention, the RTCVD chamber with reduced residual water vapor is used in a method for selective deposition of HSG. HSG is formed on a wafer surface by nucleation of Si grains on an amorphous silicon (a-Si) substrate and subsequent diffusion of Si atoms into the grain nuclei, resulting in conversion of the a-Si to a HSG structure. The inventors have found that formation of the HSG competes with other processes that take place on the wafer surface, which are dependent on the presence of water molecules. When the partial pressure of residual water vapor in the chamber is sufficiently low, preferably below 10xe2x88x928 torr, the HSG is formed with substantial selectivity relative to the competing processes.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for selectively depositing hemispherical grained silicon on the surface of a wafer in a process chamber, including:
evacuating the chamber so that a partial pressure of water vapor in the chamber is less than 10xe2x88x927 torr;
introducing a process gas mixture including silicon into the chamber;
seeding the surface with silicon grains; and
annealing the wafer.
Preferably, evacuating the chamber includes evacuating to a partial pressure of water vapor less than 10xe2x88x928 torr, and most preferably less than or equal to 10xe2x88x929 torr.
Preferably, evacuating the chamber includes pumping the chamber using a water vapor pump together with a turbomolecular pump. Preferably, the water vapor pump includes a cryogenic pump. Alternatively, the water vapor pump includes a getter, which is preferably replaced after a period of use thereof.
Preferably, the method includes regenerating the water vapor pump after a period of use thereof, most preferably by heating the water vapor pump while pumping with the turbomolecular pump.
Preferably, pumping the chamber includes using the water vapor pump to collect water vapor from the chamber, without collecting a substantial amount of the process gas in the water vapor pump.
In a preferred embodiment, introducing the process gas mixture includes introducing a mixture including at least one of the gases SiH4 and Si2H6, together with at least one of the gases N2 and H2. Preferably, introducing the mixture includes introducing a mixture at a pressure generally between 10xe2x88x926 torr and 1 torr, most preferably generally between 1xc3x9710xe2x88x925 and 5xc3x9710xe2x88x925 torr.
Preferably, annealing the wafer includes annealing at a temperature between about 600 and 700xc2x0 C. 
There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for selectively depositing hemispherical grained silicon on the surface of a wafer, including:
a process chamber, into which the wafer is introduced, the chamber having an outlet;
a turbomolecular pump having an inlet in communication with the outlet of the chamber, which pump evacuates the chamber through the inlet; and
a water vapor pump, in communication with the outlet of the chamber and the inlet of the turbomolecular pump, which water vapor pump removes water vapor from the chamber to a partial pressure less than 10xe2x88x927 torr prior to deposition of the silicon.
Preferably, the water vapor pump removes the water vapor to a partial pressure less than 10xe2x88x928 torr, and most preferably to a partial pressure less than or equal to 10xe2x88x929 torr. Preferably, the water vapor pump removes the water vapor without collecting a substantial amount of process gas from the chamber.
Preferably, the water vapor pump includes a cryogenic pump.
Alternatively, the water vapor pump includes a getter. Preferably, the getter is replaceable, and the apparatus includes a cutoff valve connected between the getter and the inlet of the turbomolecular pump, which valve is closed in order to replace the getter.
Preferably, the apparatus includes an isolation valve connected to the outlet of the chamber, which valve is closed during a regeneration cycle of the water vapor pump. Preferably, the water vapor pump is regenerated by pumping out the water vapor pump using the turbomolecular pump.
Further preferably, the apparatus includes a dry type backing pump which receives an exhaust of the turbomolecular pump.